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//! This module exists to isolate [`RandomState`] and [`DefaultHasher`] outside of the
//! [`collections`] module without actually publicly exporting them, so that parts of that
//! implementation can more easily be moved to the [`alloc`] crate.
//!
//! Although its items are public and contain stability attributes, they can't actually be accessed
//! outside this crate.
//!
//! [`collections`]: crate::collections
#[allow(deprecated)]
use super::{BuildHasher, Hasher, SipHasher13};
use crate::cell::Cell;
use crate::fmt;
use crate::sys;
/// `RandomState` is the default state for [`HashMap`] types.
///
/// A particular instance `RandomState` will create the same instances of
/// [`Hasher`], but the hashers created by two different `RandomState`
/// instances are unlikely to produce the same result for the same values.
///
/// [`HashMap`]: crate::collections::HashMap
///
/// # Examples
///
/// ```
/// use std::collections::HashMap;
/// use std::hash::RandomState;
///
/// let s = RandomState::new();
/// let mut map = HashMap::with_hasher(s);
/// map.insert(1, 2);
/// ```
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
#[derive(Clone)]
pub struct RandomState {
k0: u64,
k1: u64,
}
impl RandomState {
/// Constructs a new `RandomState` that is initialized with random keys.
///
/// # Examples
///
/// ```
/// use std::hash::RandomState;
///
/// let s = RandomState::new();
/// ```
#[inline]
#[allow(deprecated)]
// rand
#[must_use]
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
pub fn new() -> RandomState {
// Historically this function did not cache keys from the OS and instead
// simply always called `rand::thread_rng().gen()` twice. In #31356 it
// was discovered, however, that because we re-seed the thread-local RNG
// from the OS periodically that this can cause excessive slowdown when
// many hash maps are created on a thread. To solve this performance
// trap we cache the first set of randomly generated keys per-thread.
//
// Later in #36481 it was discovered that exposing a deterministic
// iteration order allows a form of DOS attack. To counter that we
// increment one of the seeds on every RandomState creation, giving
// every corresponding HashMap a different iteration order.
thread_local!(static KEYS: Cell<(u64, u64)> = {
Cell::new(sys::hashmap_random_keys())
});
KEYS.with(|keys| {
let (k0, k1) = keys.get();
keys.set((k0.wrapping_add(1), k1));
RandomState { k0, k1 }
})
}
}
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
impl BuildHasher for RandomState {
type Hasher = DefaultHasher;
#[inline]
#[allow(deprecated)]
fn build_hasher(&self) -> DefaultHasher {
DefaultHasher(SipHasher13::new_with_keys(self.k0, self.k1))
}
}
/// The default [`Hasher`] used by [`RandomState`].
///
/// The internal algorithm is not specified, and so it and its hashes should
/// not be relied upon over releases.
#[allow(deprecated)]
#[derive(Clone, Debug)]
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
pub struct DefaultHasher(SipHasher13);
impl DefaultHasher {
/// Creates a new `DefaultHasher`.
///
/// This hasher is not guaranteed to be the same as all other
/// `DefaultHasher` instances, but is the same as all other `DefaultHasher`
/// instances created through `new` or `default`.
#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
#[inline]
#[allow(deprecated)]
#[rustc_const_unstable(feature = "const_hash", issue = "104061")]
#[must_use]
pub const fn new() -> DefaultHasher {
DefaultHasher(SipHasher13::new_with_keys(0, 0))
}
}
#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
impl Default for DefaultHasher {
/// Creates a new `DefaultHasher` using [`new`].
/// See its documentation for more.
///
/// [`new`]: DefaultHasher::new
#[inline]
fn default() -> DefaultHasher {
DefaultHasher::new()
}
}
#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
impl Hasher for DefaultHasher {
// The underlying `SipHasher13` doesn't override the other
// `write_*` methods, so it's ok not to forward them here.
#[inline]
fn write(&mut self, msg: &[u8]) {
self.0.write(msg)
}
#[inline]
fn write_str(&mut self, s: &str) {
self.0.write_str(s);
}
#[inline]
fn finish(&self) -> u64 {
self.0.finish()
}
}
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
impl Default for RandomState {
/// Constructs a new `RandomState`.
#[inline]
fn default() -> RandomState {
RandomState::new()
}
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for RandomState {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("RandomState").finish_non_exhaustive()
}
}
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